The present invention relates to an automatic gain control apparatus and method and, more particularly, to a rapid settling automatic gain control that produces minimal signal distortion useful for a detector of a digital radio receiver.
The detector of a digital radio receiver converts the analog radio frequency (R/F) signal received at the antenna to the digital signal in which the message is encoded. Conversion of the signal from analog to digital format is performed by an analog-to-digital (A/D) converter in the detector. While the amplitude of the received R/F signal can vary substantially, the amplitude variation of the signal at the input to the A/D converter must be narrowly controlled because the data detection circuitry requires a relatively constant amplitude input to avoid clipping and loss of data. The input to the A/D convertor is controlled by an automatic gain control (AGC) loop.
The Institute of Electrical and Electronic Engineers (IEEE), DRAFT SUPPLEMENT TO STANDARD FOR INFORMATION TECHNOLOGY—TELECOMMUNICATIONS AND INFORMATION EXCHANGE BETWEEN SYSTEMS—LOCAL AND METROPOLITAN AREA NETWORKS—SPECIFIC REQUIREMENTS—PART 11: WIRELESS LAN MEDIUM ACCESS CONTROL (MAC) AND PHYSICAL LAYER (PHY) SPECIFICATIONS: HIGH SPEED PHYSICAL LAYER IN THE 5 GHz BAND, IEEE P802.11a/D7.0, July 1999, is a draft of a part of a family of standards for wireless Local and Metropolitan Area Networks (hereinafter, LAN). The proposed standard specifies certain characteristics of a high speed, digital, wireless communication LAN based on Orthogonal Frequency Division Multiplexing (OFDM) and packet switching. The wireless IEEE 802.11a LAN will have a data payload capacity up to 54 Mbit/sec.
In an IEEE 802.11a LAN, data is transferred in data units or frames that include a header and a data section. The header of each data unit includes a preamble field comprising a “short training sequence” and a “long training sequence.” The “long training sequence” comprising two 3.2 μsec. duration symbols is used for channel estimation and fine frequency acquisition by the receiver. The short training sequence comprises ten repetitions of a 0.8 μsec. duration symbol for a total sequence length of 8 μsec. The short training sequence is used for automatic gain control (AGC) convergence, antenna diversity selection, timing acquisition, and coarse frequency acquisition by the radio receiver. The AGC circuit must be able to converge or settle within the duration of two—three symbols if there is to be sufficient time in the short training sequence for completion of the remaining operations.
An A/D converter 10 and an associated AGC loop 12 (indicated by a bracket) of a digital radio detector are illustrated in FIG. 1. An analog R/F signal (x(t)) 14 from an antenna and RF amplifier (not illustrated) is converted to a digital signal (y(t)) 16 by the A/D converter 10. In a detector, an output signal level detector (not illustrated) senses whether the signal to noise ratio of the digital signal 16 has reached a predetermined threshold value. The automatic gain control (AGC) loop 12 controls the input 18 to the A/D converter 10 to prevent overshooting or undershooting the converter 10 as the power of the input R/F signal 14 fluctuates over the dynamic range of the system. The power of the signal at the output (y(t))16 of the A/D converter 10 is sensed at a sensor 20. The output signal power is squared in a squaring unit 22. A negative threshold value or bias (bk) 24 is added to the squared output signal power in a summer 26 to produce an error signal (e) 28. The error signal 28 is input to an integrator 30 that produces a gain voltage (v) 32 at the output. The gain voltage 32 is input to an attenuator 34 that produces a gain control characteristic or loop gain (g) 36. The amplitude of the A/D converter input (g(v)x(t)) 18 is controlled by multiplying 38 the received R/F signal (x(t)) 14 by the gain characteristic (g) 36. AGC loops are limited in the range of input signals over which a constant output amplitude can be maintained. For a given input signal dynamic range, the behavior of an AGC loop is a compromise between the time required for the loop to settle and the distortion of the signal by the loop. A high loop gain reduces the settling time. However, a high loop gain distorts the signal leading to a loss of signal to noise ratio for the detector. Obtaining the rapid settling time required by an IEEE 802.11a LAN with an acceptable signal distortion has not proven to be feasible with the classic AGC loop illustrated in FIG. 1.
FIG. 2 illustrates an A/D converter 40 and an associated improved AGC loop 42 (indicated by a bracket) providing improved transient response when compared to the classic AGC illustrated in FIG. 1. The improved AGC 42 includes a mode switch 44 that is used to switch between a closed loop operating mode and an open loop operating mode. In the closed loop mode, a control 46 sets the switch 44 to the closed loop position 48 illustrated in FIG. 2 and the circuit operates essentially as described above. However, when the R/F signal (x(t)) 50 is initially applied to the AGC it is likely to include transient signal components. During initial operation, the control 46 causes the mode switch 44 to select to the open loop mode position 52 and a predetermined initial value of the gain voltage (ζ(t)) 54 is input (γ) 56 to the attenuator 58. In the open loop mode, the input 60 to the A/D convertor 40 is not dependent upon the power of the output signal 62 which may be influenced by the transient signal components. When the control 46 senses that the gain control voltage (v) 64 at the output of the integrator 66 has approached the open loop control voltage (ζ) 54 the control 46 selects the closed loop position 48 of the mode switch 44 and the AGC switches to closed loop operation. The transient response of the classic closed loop AGC is improved by making the response independent of the power of the output signal while the output is affected by transient signal components and by providing a definite, predetermined form for the output. Further improvement in AGC performance may be obtained by storing an integrator value used for processing a prior reception of the signal in a memory 68 and using that value for a later reception. While the improved AGC provides better transient response, neither the classic AGC nor the improved AGC loop provides the rapid settling required to settle the AGC within the duration of the initial symbols of the preamble of the OFDM signal of the IEEE 802.11a LAN with an acceptable level of signal distortion.
What is desired, therefore, is an automatic gain control capable of rapid settling with minimal signal distortion.